1. Field of the Invention
The present invention relates to the field of light emitting diode (LED) power source device, and more particularly to a multi-segment LED driving circuit that adjusts the operating status of each LED string according to a change of voltage value of an AC input voltage to achieve the effects of high energy conversion efficiency and high light utilization.
2. Description of the Related Art
After LED hits the market by its features of low power consumption and high performance, it is a main subject for related manufacturers to find a way of controlling the illumination brightness, operating efficiency and service life of LED lamps. At present, most LED lamps adopt a control circuit with a constant current architecture, and the LEDs are serially connected to an N-type metal oxide semiconductor field effect transistor (N-MOSFET) and a current resistor to restrict a constant drive current passing through the LEDs by the current resistor. The current resistor receives the drive current and has a voltage drop value formed at both ends of the current resistor and fed back to an operational amplifier. After the operational amplifier compares the voltage drop value with a reference voltage value, a negative feedback circuit formed by connecting the operational amplifier, the N-MOSFET and the current resistor is provided for stepping down the voltage at both ends of the current resistor and maintaining the voltage constant and equal to the reference voltage value, so as to maintain the drive current at a constant status. In the meantime, the total harmonic distortion (THD) is restricted within a range to facilitate selling the product to markets at different places.
However, not all of the AC voltages used in different countries are the same, and the drive current and the THD also varies with the change of voltage value of the AC voltage. The greater the output power, the higher is the level of difficulty of controlling the THD within a range stably. As a result, the product quality is unstable and the product cannot be introduced into some of the markets, and the economic values and benefits are reduced. Now, if safety components are installed additionally to stabilize the THD, the cost of the lamps will be increased and unfavorable for the economic benefits.
Therefore, it is a main subject of the present invention to drive a corresponding quantity of LED strings according to the voltage change of the inputted AC current under the application condition of connecting the LEDs in series with one another to stabilize the operation quality and THD, while reducing the unnecessary power consumption and lowering the operating temperature of the overall circuit.